Molten carbonate fuel cells employ an electrolyte which is solid at room temperature and liquid or molten at operating temperatures which generally range between 500.degree. and 700.degree. C. These electrolytes include binary alkali metal carbonate compositions such as binary lithium-potassium carbonate compositions (Li.sub.2 CO.sub.3 -K.sub.2 CO.sub.3). An example of one composition is the commonly used eutectic composition consisting essentially of 62 mole % (m %) Li.sub.2 CO.sub.3 -38 mole % (m %) K.sub.2 CO.sub.3.
In most applications a plurality of fuel cells are stacked together to form a fuel cell stack. The fuel cells are joined in series to increase the power output of the stack and to maximize the output voltage of the stack. As a result of forming the stack, a voltage gradient exists along the stack from one end of the stack to the other. This voltage gradient may result in a migration of lithium and potassuim ions toward the negative end of the stack and carbonate ions toward the positive end of the stack along any path which permits movement of the ions. Because of differences in mobility between the lithium and potassium ions in the eutectic composition, the lithium and potassium ions tend to move at different rates along the length of the stack such that the molar ratio between lithium and potassium ions varies along the length of the stack. If the variation in molar ratio is great, the electrolyte in certain cells may have properties that differ markedly from the properties of the electrolyte in other cells in the stack.
The electrolyte in one cell may have reactant solubilities, ionic conductivities, melting point and other properties that are harmful to the performance of the cell during steady operation. In addition, changes in melting point can result in harmful effects during operation of the stack at a reduced temperature and during nonsteady operation, such as during stack shutdown.
Accordingly, it is important to develop an electrolyte in which the mobilities of the lithium and potassium ions are nearly equal to avoid large local variations in the molar ratio of lithium to potassium ions and to keep the molar ratio uniform along the cell stack.